Cancer is the second most common cause of death in the U.S., exceeded only by heart disease, and accounts for 1 of every 4 deaths. Since 1990, in the U.S. alone, nearly five million lives have been lost to some form of cancer.
For example, breast cancer affects 186,000 women annually in the U.S., and the mortality rate of this disease has remained unchanged for 50 years. Surgical resection of the disease through radical mastectomy, modified radical mastectomy, or lumpectomy remains the mainstay of treatment for this condition. Unfortunately, a high percentage of those treated with lumpectomy alone will develop a recurrence of the disease.
Lung cancer is the most common cause of cancer death in both sexes in the United States. Lung cancer can result from a primary tumor originating in the lung or a secondary tumor which has spread from another organ such as the bowel or breast. Primary lung cancer is divided into three main types; small cell lung cancer; non-small cell lung cancer; and mesothelioma. There are three types of non-small cell lung cancer: squamous cell carcinoma, adenocarcinoma, and large cell carcinoma. Mesothelioma is a rare type of cancer that affects the covering of the lung called the pleura, and is often caused by exposure to asbestos.
Ovarian cancer accounts for about 3% of all cancers among women and ranks second among gynecologic cancers, following cancer of the uterine corpus. Ovarian cancer affects over 20,000 women in the United States each year and causes some 15,000 deaths annually. If the disease is diagnosed at the localized stage, the 5-year survival rate is over 90%; however, only about 19% of all cases are detected at this stage.
The incidence of pancreatic cancer has been increasing steadily in the past twenty years in most industrialized countries, exhibiting the characteristics of a growing epidemiological problem.
Leukemia is a type of cancer that affects blood cells. Among the currently prescribed treatment regimes for leukemia are total body irradiation and chemotherapy. The two treatment regimes, however, pose a clinical dilemma: because leukemia is a cancer of the blood, all of the cells in the blood and all of the cells that arise in bone marrow must be treated in order to ensure destruction of the neoplastic cells. Destruction of all these cells leaves the patient in a severely immunodepressed state which could be as fatal as the leukemia.
Some cancer drugs are metabolized by an organism's naturally occuring enzymes such as adenosine deaminase (ADA, EC 3.5.4.4) and cytidine deaminase (CDA, also termed cytosine nucleoside deaminase, cytidine aminohydrolase, or EC 3.5.4.5). These enzymes function to deaminate natural aminopurine and aminopyrimidine nucleosides, respectively, in human and other organisms. These enzymes also convert active nucleoside-based cancer drugs into inactive metabolites. For example, the purine nucleoside drug arabinosyladenine (fludarabine, ara-A) is deaminated by ADA; the resulting compound, with the parent amino group replaced with hydroxyl, is inactive as an antitumor agent compared to the parent compound. Similarly, the antileukemia drug arabinosylcytosine (also termed cytarabine, Ara-C (or AraC); 4-Amino-1-(β-D-arabinofuranosyl)-2(1H)-pyrimidinone; Cytosine arabinoside; or 1-(β-D-Arabinofuranosyl)cytosine) is metabolically degraded by CDA into inactive arabinosyluracil.
CDA is a component of the pyrimidine salvage pathway. It converts cytidine and deoxycytidine to uridine and deoxyuridine, respectively, by hydrolytic deamination (Arch. Biochem. Biophys. 1991, 290, 285-292; Methods Enzymol. 1978, 51, 401-407; Biochem. J. 1967, 104, 7P). It also deaminates a number of synthetic cytosine analogs which are clinically useful drugs, such as ara-C mentioned above (Cancer Chemother. Pharmacol. 1998, 42, 373-378; Cancer Res. 1989, 49, 3015-3019; Antiviral Chem. Chemother. 1990, 1, 255-262). Conversion of the cytosine compounds to the uridine derivatives usually confers loss of therapeutic activity or addition of side-effects. It has also been shown that cancers that acquire resistance to cytosine analog drugs often overexpress CDA (Leuk. Res. 1990, 14, 751-754). Leukemic cells expressing a high level of CDA can manifest resistance to cytosine antimetabolites and thereby limit the antineoplastic activity of such therapeutics (Biochem. Pharmacol. 1993, 45, 1857-1861).
Tetrahydrouridine (THU, or 1(β-D-Ribofuranosyl)-4-hydroxytetrahydropyrimidin-2(1H)-one) has been known as an inhibitor of cytidine deaminase for a number of years.

Various reports have suggested that co-administration with THU increases the efficacy and oral activity of cytidine-based drugs. For example, THU has been shown to enhance the oral activity of anti-leukemic agent 5-azacytidine (also termed AzaC, 4-Amino-1-(β-D-ribofuranosyl)-1,3,5-triazin-2(1H)-one; or 1-(β-D-Ribofuranosyl)-5-azacytosine) in L1210 leukemic mice (Cancer Chemotherapy Reports 1975, 59, 459-465). The combination of THU plus 5-azacytidine has also been studied in a baboon sickle cell anemia model (Am. J. Hematol. 1985, 18, 283-288), and in human patients with sickle cell anemia in combination with orally administered 5-azacytidine (Blood 1985, 66, 527-532).
THU has also been shown to enhance the oral efficacy of ara-C in L1210 leukemic mice (Cancer Research 1970, 30, 2166; Cancer Invest 1987, 5, (4), 293-9), and in tumor-bearing mice (Cancer Treat. Rep. 1977, 61, 1355-1364). The combination of intravenously-administered ara-C with intravenously-administered THU has been investigated in several clinical studies in humans (Cancer Treat. Rep. 1977, 61, 1347-1353; Cancer Treat. Rep. 1979, 63, 1245-1249; Cancer Res. 1988, 48, 1337-1342). In particular, combination studies in patients with acute myeloid leukemia (AML) and chronic myeloid leukemia (CML) have been performed (Leukemia 1991, 5, 991-998; Cancer Chemother. Pharmacol. 1993, 31, 481-484).
Gemcitabine (also termed dFdC; 1-(4-Amino-2-oxo-1H-pyrimidin-1-yl)-2-deoxy-2,2-difluoro-β-D-ribofuranose; or 2′-deoxy-2′,2′-difluorocytidine; or 2′,2′-difluoro-2′-deoxycytidine), another cytidine-based antineoplastic drug, has also been studied in conjunction with CDA inhibitors (Biochem. Pharmacol. 1993, 45, 1857-1861). Co-administration with THU has been shown to alter the pharmacokinetics and bioavailability of gemcitabine in mice (Abstr. 1556, 2007 AACR Annual Meeting, Apr. 14-18, 2007, Los Angeles, Calif.; Clin. Cancer Res. 2008, 14, 3529-3535).
5-Fluoro-2′-deoxycytidine (fluorocytidine, FdCyd) is another cytidine-based anticancer drug which is an inhibitor of DNA methyltransferase. The modulation of its metabolism and pharmacokinetics by THU in mice has been studied (Clin Cancer Res., 2006, 12, 7483-7491; Cancer Chemother. Pharm. 2008, 62, 363-368). FdCyd in combination with THU is currently the subject of an ongoing clinical trial identified by National Cancer Institute clinical trial no. NCT00378807.
The results of the aforementioned studies suggest that there is therapeutic utility in the administration of CDA inhibitors together with cytidine-based drugs such as gemcitabine, ara-C, 5-azacytidine and others. However, early CDA inhibitors such as THU suffer from drawbacks that include acid instability (J. Med. Chem. 1986, 29, 2351) and poor bioavailability (J. Clin. Pharmacol. 1978, 18, 259).
Thus, there is therefore an ongoing need for new, potent and therapeutically useful inhibitors of CDA, and new compositions that are useful for treating cancer or neoplastic disease.